Concrete is a strong and durable material, but it has its limits. Cracking is a common issue that can occur when the shrinkage forces are greater than the strength of the concrete. This is usually due to the evaporation of some of the water contained in the concrete during hardening. Other causes of cracking include excessive weight, poor curing, lack of proper reinforcement, and plastic shrinkage.
When concrete is still in its plastic state, it is filled with water which takes up space and makes the slab a certain size. As the slab loses moisture during curing, it becomes a little smaller and this shrinkage creates stress in the slab. If the stress becomes too great for the now hardened concrete, the slab cracks to relieve stress. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished.
Settlement cracks typically occur when a void is created in the ground below the concrete surface. Differential thermal deformations can also cause cracking due to temperature differences between the top and bottom of the slab. To prevent cracking, it is important to understand what your contractor is doing with respect to each of the items listed above. Careful timing is essential to ensure that the concrete liquefies again under the action of the vibrator and that the cracks are completely closed.
Wet curing can also help maintain the curing process by keeping the newly laid concrete wet. It is also important to reduce the pH of concrete by carbonation or by preventing chlorides from entering it. This will help protect steel from corrosion and reduce peeling of joints which can reduce the life of a concrete structure. By following these procedures and exercising proper care, you will get the best performance from your concrete and prevent cracking.